1. Field of the Invention
The present invention relates to a single cell for a solid oxide fuel cell and a solid oxide fuel cell using the same, more particularly to a single cell for a solid oxide fuel cell and a solid oxide fuel cell using the same which are preferably used for a distributed power source, a cogeneration system or the like in urban areas.
2. Description of Related Art
A solid oxide fuel cell (hereinafter referred to as an “SOFC”) is a fuel cell in which a solid electrolyte showing oxide ion conductivity is used as an electrolyte. In the SOFC, since the electrolyte is solid, there is no problem of dissipation of the electrolyte, and long life can be expected. Further, as an operating temperature is as high as about 1000° C., the utility value of waste heat is also high. Furthermore, as an output power density is high, the SOFC can be expected to be compact and of high efficiency.
In general, cell structures of the SOFC are broadly divided into those of planar type, tubular type and integral type. Among them, the planar type SOFC has advantages of high power density as internal resistance is comparatively low, and of high output power density per unit area because thin cells are laminated.
The planar type SOFC is further broadly divided into those of a self-supporting electrolyte film type and of a supported electrolyte film type. The former one has a structure in which a plurality of single cells are laminated via a separator, wherein both sides of a plate solid electrolyte are bonded to a fuel electrode brought into contact with a fuel gas such as hydrogen and city gas, and to an air electrode brought into contact with an oxidant gas such as air and oxygen.
On the other hand, the latter one has a structure in which a plurality of single cells are laminated via a separator, wherein a very thin solid electrolyte film is supported by a thick fuel electrode, and a thin air electrode is bonded to the other side of the thin solid electrolyte film.
In the SOFC with such a constitution, if the fuel electrode and the air electrode are provided with the fuel gas and the oxidant gas, respectively, since there exists a difference between oxygen partial pressure on the air electrode side and that on the fuel electrode side, the oxygen ionizes at the air electrode and is carried through the solid electrolyte to the fuel electrode, then the oxide ion at the fuel electrode reacts with the fuel gas to emit an electron. Therefore, if a load is connected to the fuel and air electrodes, it becomes possible to directly take out free energy change of the cell reaction as electrical energy for power generation.
Incidentally, for the solid electrolyte used in the above-mentioned SOFC, yttria-stabilized zirconia (hereinafter referred to as “YSZ”) is conventionally used. However, YSZ is of high internal resistance and shows low oxide ion conductivity. Therefore, recently, in view of improving the output power density, lowering the operating temperature or the like of SOFC, attention has been directed at scandia-stabilized zirconia (hereinafter referred to as “ScSZ”), and a variety of researches are conducted.
For example, Japanese Patent Application Unexamined Publication No. Hei6-107462 discloses a technique relating to a single cell for an SOFC, and an SOFC in which a solid electrolyte is scandia and alumina-stabilized zirconia prepared by substituting a part of Sc in ScSZ which contains Sc as a main dopant with Al with stable tervalence for maintaining the oxide ion conductivity higher than that of YSZ and for suppressing phasetransition.
In addition, generally, for the fuel electrode, a cermet of Ni and YSZ containing 8 mol % of Y2O3 (hereinafter referred to as “Ni-8YSZ”) or the like is used. For the air electrode, (La, Sr) MnO3 being perovskite type transition metal oxide or the like is used. For the separator, LaCrO3 or the like is used.
However, even if a single cell for an SOFC and an SOFC are constituted by using the above-mentioned materials as a solid electrolyte, a fuel electrode, an air electrode and a separator (in the case of the SOFC), it cannot be necessarily said that a single cell for an SOFC and an SOFC having practicable generating performance and durability are directly obtained.
That is because, since the single cell and the SOFC are used while being bonded to or in contact with different types of constituting materials at the times of manufacture, fabrication and operation, the cell performance is greatly influenced not only by material properties exhibited by each constituting material such as the oxide ion conductivity of the solid electrolyte or mechanical characteristics such as strength and toughness, or electrode activity of the fuel electrode and the air electrode or the like, but also by an interaction between the different types of constituting materials such as a bonding state of the solid electrolyte to both of the electrodes or a contact state of each electrode with the separator. Therefore, unless a comprehensive development is performed in view of those characteristics, it can be said that a single cell for an SOFC and an SOFC capable of developing excellent generating performance and durability as a cell are not obtained.
Consequently, in the case of putting the single cell and the SOFC into practical use at an early stage, it is specifically important to improve the cell performance of the single cell itself being a fundamental unit structure, and not to degrade the cell performance in the case of stacking the single cells. That is because, depending on the constitution of the single cell, the generating performance and the durability developed by the single cell becomes entirely alien, and as a result, the generating performance and the durability of the SOFC when a plurality of the single cells are stacked are greatly influenced.